This invention relates to a motor control apparatus and, more particularly, to a motor control apparatus capable of preventing the dead zone of a pulse-width modulating circuit from causing a short circuit in an inverter circuit.
A control circuit for an AC or DC motor generally makes use of an arrangement, namely a pulse-width modulation drive circuit, for pulse-width modulating an AC or DC current command to drive an inverter circuit (a transistorized amplifier circuit) and for applying a pulse-width modulated sinusoidal or direct current to a motor. It is sufficient if the arrangement is capable of withstanding the high voltage required solely by the final stage of the inverter circuit, the circuit construction is thus simple and control is performed comparatively well.
In the case of direct current the inverter circuit has four transistors. For three-phase AC drive the inverter circuit has two transistors in each phase, for a total of six transistors, with control being such that when one of the transistors in each phase turns on, the other turns off. A transistor drive signal (a pulse-width modulated signal) is provided with dead time in such a manner that the two transistors in each phase will not be turned on simultaneously at the time of the changeover, that is, in such a manner that the DC power supply will not develop a short circuit. This dead time is set by providing a pulse-width modulating circuit with a dead zone. However, since the pulse-width modulated signal is provided with the dead time in this manner, the pulse width of the modulated signal is reduced and distortion is produced in the pulse-width modulated sinusoidal current or in the direct current. This results in certain disadvantages, namely greater excitation noise and torque irregularity, with the commanded torque originally not being obtained. In particular, in the case of operation at low speed, the sinusoidal current command takes on a low frequency, and pulse width is small with a direct current command of a low voltage. Accordingly, the influence of the dead zone is of such proportions as cannot be ignored.
As means for compensating for the dead zone there has been proposed a system in which the output voltage of the inverter circuit is fed back to a pre-stage of the pulse-width modulating circuit to raise the gain using this feedback loop. However, with an arrangement in which an arithmetic circuit such as a microprocessor is used as the motor control circuit, it is necessary to provide separate analog circuits for these feedback loops. The processing of these analog circuits cannot be performed by the arithmetic circuit, thereby resulting in a complicated construction and high cost.